This invention relates to gasket sheet materials, particularly of the type having a corrosion-resistant surface coating on the outside surfaces.
Gasket sheet materials of the prior art generally include a flat steel sheet as the core material and a joining sheet (ASTM F104 "F112000" or ASTM F104 "192000"), which is a compound of asbestos or non-asbestos fibers and synthetic rubber as a surface elastomer. Also as a surface material a beater sheet (ASTM F104 "F125000"), impregnated, spray coated, or roll coated with fluorine or a silicone phenol resin is used.
Gasket sheets made of a joint sheet have an advantage of better stress relaxation properties; however, they have the disadvantage of poor conformability because of their hard surface material. Moreover, due to insufficient impermeability of the surface material, the mechanical properties of the gasket sheet, such as tensile strength, tear strength and bending strength, are affected adversely. In addition, it has been found that the binder in the surface material disintegrates from chemical attack causing damage to the surface material due to corrosion and/or unwanted adhesion.
While the gasket sheets made of a beater sheet have the advantage of better conformability, they show rather poor stress relaxation properties. Surface treatment of gasket sheet is necessary to improve the stress relaxation properties. Spray coating, which is applied to the beater sheet gasket to improve its permeability, has the disadvantage that much of the coating agent is wasted since its recovery is very difficult. Roll coating has also a disadvantage that the coating cannot be made thick because of agent transcription by the rolls to which the composite is subjected.